The present invention is directed to an iron base, heat and corrosion resistant alloy that has low density, good tensile ductility, and excellent properties related to oxidation resistance, corrosion resistance, castability and strength. This new class of alloys is about 20-25% lighter and 20-80% cheaper than most traditional nickel-containing steels, e.g., stainless steels, heat resistant steels and heat resistant alloys.
Currently, heat resistant structural applications most often employ heat resistant steels, heat resistant alloys and superalloys. There is, however, a need for materials with similar properties having a much lower density since heat-resistant steels, heat-resistant alloys, and superalloys have relatively high densities. While alternative materials such as ceramics and intermetallic ordered alloys are being studied for their low densities, none of them have achieved the combination of low density, adequate tensile ductility, high strengths, and good oxidation resistance that is needed for high temperature engineering applications.
In the case of ceramics, their complete lack of tensile ductility severely limits the advantage of their low densities. In addition, ceramic components are usually produced through a powder sintering process which is a relatively costly process. Because of their lack of ductility and high cost, ceramics parts can only be used in very limited applications.
Light intermetallic ordered materials have not achieved adequate intrinsic tensile ductility and exhibit low fracture toughness, especially at room temperature. As a result of these properties, relatively complex processing techniques have to be employed to produce these materials and fabricate them into components. This significantly increases the production costs and their relatively low toughness at room temperature can cause handling problems and high component rejection rates.
An example of such an intermetallic ordered material is Fe3Al. Unlike pure iron, which is a body centered cubic (BCC) solid solution and is very ductile, Fe3Al forms an ordered BCC structure (generally defined as DO3 at room temperature and B2 at high temperatures) in which Fe atoms and Al atoms are arranged in a regular fashion. Fe3Al has a low density and reasonably good oxidation resistance up to about 800xc2x0 C. because of its high aluminum content. The aluminum in the material will easily form an oxide scale in an oxidizing environment, although the oxide scale is not strong and easily spalls at temperatures above 800xc2x0 C. Moreover, the raw materials for Fe3Al are also relatively inexpensive. However, Fe3Al is very brittle and has a low room temperature tensile ductility, it easily fractures in both intergranular and transgranular fashion.
Although chromium containing Fe3Al has shown limited improvement in tensile ductility and is relatively lightweight, as evidenced by a density of about 6.5 g/cm3, conventional ordered Fexe2x80x94Alxe2x80x94Cr compositions suffer from relatively poor high-temperature strengths, corrosion resistance and oxidation resistance.
Consequently, the simultaneous achievement of a more affordable heat resistant structural material that has a low density, good tensile ductility, excellent oxidation resistance and excellent workability, is a continuing objective of this field of endeavor. Specifically, there has been a need for a new iron-base alloy having a low density, high strength, adequate tensile ductility, defined as xe2x89xa75% tensile elongation, and excellent oxidation and corrosion resistance. The above-mentioned objectives can be substantially realized by adding carbon to a chromium-containing iron aluminum compound such that a body-centered-cubic iron aluminum chromium carbon alloy is formed.
The immediate application for the present invention includes turbochargers for high speed diesel engines used in boats, trucks and passenger cars. Diesel engines are widely used because of better fuel economy than gasoline engines. To achieve such fuel economy, as well as increase engine efficiency and reduce pollution, turbo-chargers are routinely used in high-speed diesel engines. Most industrial trucks as well as about 10% of passenger cars in the world (up to 20% in Europe and 10% in Japan) are powered by high-speed diesel engines with turbochargers.
A turbocharger for a diesel engine is made up of a compressor and a turbine. From a mechanical performance perspective, the turbine is the most critical part, since it operates at high temperatures, e.g., up to 650xc2x0 C., and under high centrifugal stress due to high-speed rotation. The environment in which a turbine operates can also be both oxidizing and corrosive.
Currently, turbocharger turbines are cast from an iron-nickel base alloy or a nickel base alloy that is both expensive and heavy. Because of the weight, it takes time for present turbochargers to overcome inertia before the turbine can reach the working speed in which it operates most effectively. As evidenced by the emission of a dark cloud of exhaust on sudden acceleration, the exhaust gas is not properly burned during the time it takes for the turbine to reach its operating speed. To solve the above-mentioned problems associated with Fe-Ni base or Ni base-alloy turbochargers, turbocharger turbines and compressors from the body-centered-cubic iron aluminum chromium carbon alloy have been fabricated of the present invention.
Accordingly, a subject of the present invention is a material comprising a body-centered-cubic, single-phase, solid solution of iron aluminum, specifically Fexe2x80x94Alxe2x80x94Crxe2x80x94C. Preferably the material includes about 10 to 80 at. % iron, about 10 to 45 at. % aluminum, about 1 to 70 at. % chromium and about 0.9 to 15 at. % carbon. The material has excellent properties in polycrystalline form. In addition, the material can be strengthened by well-known methods that include solid solution strengthening, grain size refinement or by the introduction of particles of a strengthening phase. Preferably, the material can be strengthened by precipitating within the solid solution, BCC, solid solution particles that have substantially the same lattice parameters as the underlying solid solution. The inventive material is oxidation resistant at temperatures up to 1150xc2x0 C., and has excellent mechanical properties at temperatures up to about 650xc2x0 C.